Hydroboration Of Alkenes Research Articles

Copper-Catalyzed Conjugate Additions of Alkylboranes to Imidazolyl α,β-Unsaturated Ketones: Formal Reductive Conjugate Addition of Terminal Alkenes

Conjugate addition of alkylboron compounds (alkyl-9-BBN) to imidazol-2-yl α,β-unsaturated ketones proceeded in the presence of a catalytic amount (10 mol %) of CuCl, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), and t-BuOK. The alkylboranes are available through alkene hydroboration, and thus the overall process represents a reductive conjugate addition of alkenes to enone derivatives. A variety of functional groups are tolerated in both the alkenes and the α,β-unsaturated ketones. The 2-acylimidazole moiety can easily be converted into the corresponding carboxylic acid, ester, and amide derivatives.

Synthesis of Macro(alkoxyamines) via Hydroboration of Polyolefins with Subsequent Nitroxide Oxidation for Controlled NMP

AbstractA novel method for the synthesis of alkoxyamines that are subsequently used as initiator/regulators for NMP is reported. This method can be applied to low‐molecular‐weight olefins as well as to macromolecules containing alkene moieties to form poly(alkoxyamines). The reaction sequence comprises hydroboration of olefins with catecholborane followed by oxidation of alkylborane intermediates using nitroxides in the presence of methyl vinyl ketone to ultimately yield alkoxyamines that serve as suitable initiators for NMP. The first application of these alkoxyamines for the preparation of comb and star polymers is presented. magnified image

Cp*IrCl 2] 2 catalyzed hydroborations of alkenes using a bulky dioxaborocine

4,8-Di- tert-butyl-2,10-dimethyl-12 H-dibenzo[ d, g][1,3,2]dioxaborocine ( 1) has been prepared in high yield by the addition of H 3B ·SMe 2 to 6,6′-methylene(2- tert-butyl-4-methylphenol). Dioxaborocine 1 is a relatively stable solid that reacts with a variety of aliphatic alkenes in the presence of catalytic amounts of [Cp*IrCl 2] 2 to give the terminal hydroboration products. Analogous reactions with vinylarenes, however, afford the corresponding alkenylboronate esters along with equal amounts of the hydrogenation products. Boron products have been characterized by a number of physical and analytical methods, including single-crystal X-ray diffraction studies.

Oxidation of Alkylcatecholboranes with Functionalized Nitroxides for Chemical Modification of Cyclohexene, Perallylated Polyglycerol and of Poly(butadiene)

AbstractThe present communication reports on the synthesis of alkoxyamines by hydroboration of olefins with catecholborane and subsequent oxidation by using nitroxides. Oxidation occurs via a radical process and the intermediately formed C‐radicals are trapped by the nitroxides to form the corresponding alkoxyamines. If functionalized nitroxides are used, the method allows incorporation of interesting functional moieties via this approach. The novel method is applied for chemical modification of cyclohexene as a test substrate. More importantly, it is also shown that the reaction sequence can be used for chemical modification of macromolecules containing multiple double bonds. This is documented by successful functionalization of poly(butadiene) and of perallylated polyglycerol.

Alkene Hydroboration: Hot Intermediates That React While They Are Cooling

Non-TST behavior has recently attracted a great deal of attention. If such behavior is general, then the standard way in which synthetic chemists rationalize and predict reactivity and selectivity would be at least partially invalid. The work in this article was inspired by recent results which highlighted a departure from the predictions of TST for rationalization of the regiochemical outcome of the hydroboration reaction mechanism, suggesting that the isomeric product ratios arise because of nonstatistical dynamical effects (J. Am. Chem. Soc. 2009, 131, 3130-3131). We suggest, based on new calculations using a weak collision RRKM-Master Equation (ME) model, an alternative interpretation of the experimental results which preserves a statistical reaction model. While it is a common assumption that all intermediates and transition states along the reaction path are in thermal equilibrium with solvent, our ME results show that hot intermediates may react while they are undergoing stepwise relaxation through weak collisions, even in solution. To our knowledge, this work represents the first application of master equation methodology to a solution phase thermal reaction in organic chemistry that cannot be otherwise explained using conventional TST. Explicit modeling of solvent and solute dynamics is often prohibitively expensive; however, the master equation offers a computationally tractable model with considerable predictive power that may be utilized to investigate whether stepwise collisional relaxation is prevalent in other polyatomic systems.

ChemInform Abstract: Novel Reduction of Carboxylic Acids and Hydroboration of Olefins by Electrolysis of Sodium Borohydride.

Electrochemical oxidation of sodium borohydride in diglyme containing aliphatic or aromatic carboxylic acids followed by acid-catalyzed hydrolysis afforded the corresponding primary alcohols in good yields. Furthermore, similar electrochemical oxidation in the presence of a variety of olefins instead of carboxylic acids, followed by treatment with alkaline–hydrogen peroxide, brought about novel electrochemical hydroboration to give the corresponding alcohols regio- and stereoselectively in good yields.

ChemInform Abstract: LiBH4-Promoted Hydroboration of Alkenes with 1,3,2-Benzodioxaborole.

In the presence of a small amount of LiBH4 mono-, di-, tri- and tetra-substituted ethenes were hydroborated almost quantitatively with 1,3,2-benzodioxaborole (catecholborane) under very mild conditions.

ChemInform Abstract: Lithium Triethylborohydride-Promoted Hydroboration of Alkenes with Dialkoxyboranes.

In the presence of a catalytic amount of lithium triethylborohydride (LiBEt3H) the hydroboration of alkenes with dialkoxyboranes is promoted markedly to provide the hydroboration products almost quantitavely under mild reaction conditions.

ChemInform Abstract: Rhodium(I)- and Iridium(I)-Catalyzed Hydroboration Reactions: Scope and Synthetic Applications.

A study of the rhodium(1)- and iridium(1)-catalyzed hydroboration of olefins with catecholborane is described. Applications to organic synthesis were one focus of this investigation. The scope of the reaction was defined, and issues of stereoselection were addressed. The rhodium-catalyzed hydroboration of several classes of allylic alcohols was found to be highly diastereoselective, preferentially affording the isomer complementary to that furnished by the uncatalyzed variant of the reaction (9-BBN). The first two general approaches to effecting a directed olefin hydroboration were developed. Both phosphinites and amides proved capable of delivering the transition metal reagent.

ChemInform Abstract: Nickel-Catalyzed Hydroboration with Catecholborane.

Hydroborations of alkenes and alkyne with catecholborane were found to be catalyzed by activated nickel powder.

ChemInform Abstract: Amide‐Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Dramatic Effect of Lewis Acids on the Rhodium‐Catalyzed Hydroboration of Olefins.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Stereoselective Allylic C-H Activation with Tertiary Alkylboranes: A New Method for Preparing Cycloalkyl Derivatives with Three Adjacent Stereocenters.

A regio- and stereoselective migration takes place under mild conditions for tertiary organoboranes obtained by hydroboration of tetrasubstituted cyclic alkenes. A subsequent amination proceeds with perfect control of the stereochemistry. The scheme shows an example of this for the reaction of bicyclo[4.3.0]non-1(6)-ene (1) to 2 via intermediates 3 and 4. Compound 2 is formed as the single diastereomer in 69% yield.

ChemInform Abstract: A Highly Regioselective Epoxide Ring Opening Reaction by Triorganylborane via Electrochemical Method.

We recently described that triorganylboranes can be used as organyl group transfer agents to the carbonyl compounds by implementing an electrochemical procedure that uses copper as a sacrificial anode. Secondary and tertiary alcohols are readily obtained in good yields by performing the electrochemical reaction in an undivided cell with a platinum electrode and a copper sacrificial anode. This methodology has a great attention since of its enormous potential synthetic utility. The reaction can offer the wide scopes for the further application to various functional groups similar to Grignard reagents derived from alkyl halides. Therefore, we now report the epoxide ring opening reaction by the electrochemical organyl transfer of triorganylboranes at constant negative reduction potential (-2.6 V~-3.0 V) in an undivided electrolytic cell using a sacrificial metallic anode. Especially, of particular interest is the regioselectivity for the epoxide ring opening reaction by electrochemical tool. In addition, these electrochemical epoxide ring opening reactions were compared with those of corresponding Grignard reactions. For most purposes, triorganylboranes, such as tributylborane and tri-sec-butylborane, adapted in this experiment would be derived from hydroboration of alkenes, however triphenylborane could not be prepared by hydroboration in this case Grignard reagent would be utilized. First of all, a variety of degassed solvents, such as DMF (entry 5), THF (entry 6), Py (entry 7), and acetonitrile (AN) (entry 8), sacrificial anodes, such as Cu (entry 5), Mg (entry 9), Al (entry 10), Zn (entry 11), and Fe (entry 12), and electrolytes, such as tetrabutylammonium tetrafluoroborate (Bu4NBF4) (entry 5), tetrabutylammonium perchlorate (Bu4NClO4) (entry 13), tetrabutylammonium hexafluorophosphate (Bu4NPF6) (entry 14), tetrabutylammonium chloride (Bu4NCl) (entry 15), tetrabutylammonium bromide (Bu4NBr) (entry 16), tetrabutylammonium iodide (Bu4NI) (entry 17), and lithium bromide (LiBr) (entry 18) with trisec-butylborane and styrene oxide were examined to find the combination which afforded the best yield. As a result, it revealed DMF as a solvent, Cu sacrificial anode and Bu4NBF4 as a supporting electrolyte gave the best yield to the corresponding alcohol (entry 5). Additionally, we found that the reaction time decreased with increasing their stainless steel cathode surface area from 4 h at 1 cm (entry 19) to 1.5 h at 4 cm (entry 5) with increasing of yield from 74% to 89%, and by addition of bases, such as potassium tert-butoxide (entry 5), sodium ethoxide (entry 20), sodium hydroxide (entry 21), and sodium 2,6-di-tert-butyl-4-methylphenoxide (entry 22), especially among them, by addition of potassium tert-butoxide the efficiency significantly enhanced, resulting in the reduction of reaction potential from -3.2 V to -2.6 V. Moreover, it revealed that yields of electrochemical epoxide ring opening reactions were markedly influenced by addition of Lewis acid, such as BF3 · etherate, and the electrochemical reaction of 2b by 1b in the presence of BF3 · etherate afforded the exclusively corresponding less substituted alcohol in 89% yield (entry 5) whereas in absence of BF3 · etherate gave in 57% yield (entry 23) under same circumstance. We also found that there was no significant difference in yield between Pt and stainless steel cathode in the electrochemical epoxide ring opening reaction with 1b, however, it should be noticed that the regioselectivity significantly depends upon the choice of cathode; Stainless steel furnished the exclusive product 3 with extremely high regioselectivity (entry 5), whereas Pt gave two products, 3 and 4, with somewhat low regioselectivity (entry 24) which was,

ChemInform Abstract: Enhanced Regioselectivity of Rhodium-Catalyzed Alkene Hydroboration in Supercritical Carbon Dioxide.

ChemInform Abstract: Enhanced Regioselectivity of Rhodium-Catalyzed Alkene Hydroboration in Supercritical Carbon Dioxide.

ChemInform Abstract: Efficient Synthesis of L-Altrose and L-Mannose.

Abstract Two convenient routes for the synthesis of l -altrose and l -mannose from 1,2:3,5-di- O -isopropylidene-β- l -idofuranose in four and six steps via the stereoselective hydroboration and hydrogenation of olefins as key steps are described here, respectively.

Asymmetric Hydroboration of Trisubstituted Alkenes

Asymmetric Hydroboration of Trisubstituted Alkenes

Copper-Catalyzed γ-Selective Allyl−Alkyl Coupling between Allylic Phosphates and Alkylboranes

Copper-catalyzed allyl-alkyl coupling between allylic phosphates and alkylboranes, prepared by hydroboration of alkenes with 9-BBN-H, takes place with complete gamma- and E-selectivities and with preferential 1,3-anti stereochemistry. The reaction tolerates various functional groups in both the allylic phosphate and alkylborane. Catalytic mechanisms involving transmetalation between a trialkyl(alkoxo)borate and a copper(I) complex to form an alkylcopper(I) species are proposed.

Amide-Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes

Rhodium-catalyzed hydroborations of trisubstituted alkenes are generally slow and often suffer from competing alkene isomerization. In contrast, the trisubstituted alkene moieties contained within the framework of a beta,gamma-unsaturated amide undergo facile reaction, perhaps facilitated by carbonyl directing effects and two-point binding of the substrate to the rhodium catalyst. Stereoisomeric substrates, for example, (E)- and (Z)-3, cleanly give rise to diastereomeric products, and thus the rhodium-catalyzed reaction is stereospecific. In addition, simple TADDOL-derived phenyl monophosphite ligands in combination with Rh(nbd)(2)BF(4) afford highly enantioselective catalysts (seven examples, 91-98% ee). These catalysts provide an alternative methodology to prepare Felkin or anti-Felkin acetate-aldol products and related derivatives that are obtainable from the intermediate chiral organoboranes.

Dramatic Effect of Lewis Acids on the Rhodium-Catalyzed Hydroboration of Olefins

The addition of Lewis acids such as trispentafluoroboron as cocatalysts has been found to have a dramatic effect on the Rh-catalyzed hydroboration of olefins with pinacol borane. For example, aliphatic olefins do not react at all in noncoordinating solvents, but with the addition of 2% of B(C(6)F(5))(3), the reaction is complete in minutes. Similarly, the reaction of aromatic olefins with HBPin occurs slowly and nonselectively in the absence of B(C(6)F(5))(3), but is accelerated and occurs more selectively in its presence. Preliminary mechanistic studies suggest that the B(C(6)F(5))(3) needs to be present throughout the course of the reaction, not just at the initiation stage, and implicate this species, along with THF, in the heterolytic cleavage of the B-H bond of HBPin.